In recent years, because of the global need for lower fuel consumption and less exhaust emission, electric and hybrid vehicles mounted with an AC motor as a mechanical power source are proposed. By way of example, a hybrid vehicle is mounted with a DC power supply, which may be a secondary battery, an electric power converter, which includes inverters, and AC motors. The power supply is connected to the motors via the power converter. The inverters convert the DC voltage from the power supply into AC voltages for driving the motors.
The hybrid or electric vehicle is provided with a control apparatus, which includes two or three current sensors for sensing the currents in two phases or three phases of a three-phase motor. The apparatus controls the motor according to the outputs (sensed currents) from the current sensors. Thus, it is necessary to provide two or three current sensors for each of motors. This results in a bar to simplification of the parts of the inverter that include three-phase output terminals. This also results in a bar to reduction of the cost of a motor control system of the vehicle.
Patent documents 1 (JP 2004-64903A), 2 (JP H10-225199A), 3 (JP 2001-145398A, U.S. Pat. No. 6,229,719) and 4 (JP 2004-159391A) disclose exemplary AC motor control apparatuses, which use one current sensor relative to plural phases.
In patent document 1, one current sensor is provided in a DC power supply line to sense a current flowing in a bus. If the current sensor is provided in the DC power supply line in a control system for a high power AC motor mounted in a vehicle, not only the assembling work near the DC power supply line is complicated but also extension of the DC power supply line causes current conduction noise. Thus an inverter will become large and expensive.
In patent document 2, a d-axis current Id and a q-axis current Iq are calculated from a sensed current of one phase among three phases by using a state equations determined in accordance with motor constants. According to this technology, since the motor constants vary with temperature, it is likely that an estimation error becomes large when the d-axis current Id and the q-axis current Iq are calculated by solving the state equations. Thus motor control cannot be stabilized. Further, complicated calculation processing is needed and hence it is not readily possible to implement such a technology in a control ECU (microcomputer).
In patent document 3, a d-axis current Id and a q-axis current Iq of an AC motor are calculated by d-q conversion of a sensed current of one phase among three phases and estimated phase currents of other two phases. The currents Id and Iq are averaged by first-order delay filters and inversely d-q converted to estimate phase currents of the other two phases. According to this technology, a delay is caused in the estimated current and motor control cannot be stabilized by an influence of a first-order delay filter provided for averaging, when a torque change or a rotation speed change is required as in a vehicle.
In patent document 4, one current sensor is provided for sensing a current flowing in one phase (for example, U-phase) among phases of an AC motor and currents of the other two phases (for example, V-phase and W-phase) are estimated based on a current of one phase (for example, U-phase) sensed by the current sensor, d-axis and q-axis command currents, and electrical angle information of the AC motor.
Specifically, this technique includes: determining a U-phase current phase angle θ′ (=θ+α) by adding the command current phase angle α between the q-axis and the vector resultant from the d-axis command current Id* and q-axis command current Iq* of the AC motor to the angle θ between the rotor of the motor and the U-phase axis of the stator of the motor; calculating a current amplitude Ia from the U-phase current phase angle θ′ and the sensed current Iu in the U-phase according to the following equation (A); calculating estimated currents Iv and Iw in the V-phase and W-phase respectively from the current amplitude Ia and U-phase current phase angle θ′ according to the following equations (B) and (C); calculating an d-axis estimated current Id and an q-axis estimated current Iq from the sensed current Iu in the U-phase and the estimated currents Iv and Iw in the V-phase and W-phase respectively; and performing the feedback control of the current in the AC motor by so calculating a command voltage for the motor that the estimated currents Id and Iq equal the command currents Id* and Iq* respectively.Ia=Iu/[√{square root over (⅓)}×{−sin(θ′)}]  (A)Iv=√{square root over ((⅓))}×Ia×{−sin(θ′+120°)}  (B)Iw=√{square root over ((⅓))}×Ia×{−sin(θ′+240°)}  (C)
A d-axis estimated current Id and a q-axis estimated current Iq are calculated based on the sensed current Iu of one phase and the estimated currents Iv and Iw of the other two phases. Command voltages for the AC motor are calculated and the current flowing to the AC motor is feedback-controlled so that the estimated currents Id and Iq attain the command currents Id* and Iq*. However, according the technology of patent document 4, the current amplitude Ia is calculated by dividing Iu by 0 in the equation (A) when sin(θ′) becomes 0 at a U-phase current phase angle θ′=0[°]. The current amplitude Ia thus cannot be calculated accurately and hence the other two estimated currents Iv and Iw cannot be calculated accurately either. The other two estimated currents Iv and Iw are calculated as Iv=0 [A] and Iw=0 [A] by the equations (B) and (C), respectively, when the detected current Iu becomes 0. In this case, it becomes impossible to control the AC motor.